1. Field
The disclosed concept pertains generally to systems employing semiconductor devices and, more particularly, to systems, such as, for example, power systems controlling or monitoring power semiconductor devices.
2. Background Information
Induction motor drives, also called alternating current (AC) drives, are used to control the speed and torque of multi-phase induction motors, which for a long time have been the workhorse of the industry.
AC drives can be divided into two categories: low-voltage and medium-voltage. The low-voltage AC drives are widely used and cover the 0 VAC to about 600 VAC range. Low-voltage AC drives are manufactured by almost five hundred companies around the world. Medium-voltage AC drives cover input line voltages above about 660 VAC and up to about 15,000 VAC. Only about a half-dozen known companies design and produce medium-voltage AC drives. High-voltage AC drives cover voltages of about 15,000 VAC and higher, but are very uncommon compared to low-voltage and medium-voltage AC drives. Recently, the auto industry and some other special applications providing low output voltage harmonics are considering the use of multi-level inverter bridges for low-voltage motors.
Until recently, power semiconductor switches were rated at a maximum of 1,700 V, which has allowed the low-voltage three-phase AC drives to use a six-switch inverter bridge. Today, state-of-the-art semiconductor switches are rated at 2,500 V, 3,300 V, 4,500 V, 6,500 V and can be used in a two-level, six-switch inverter bridge having up to a 2,000 VAC input. Above 2,000 VAC, the inverter bridge employs a greater number of power semiconductor switches connected in series. The most popular inverter topology for three-phase, medium-voltage induction motors of up to 4,000 V is a three-level, twelve-switch inverter bridge.
The number of levels in an inverter bridge defines the number of direct current (DC) voltage steps that are employed by the inverter bridge in order to achieve a certain voltage level in its output. Because power semiconductor switches have limited voltage capability, the total DC bus voltage of an inverter bridge is divided into a number of voltage steps, such that each voltage step can be handled by one power switch.
In a conventional two-level AC drive, three-phase AC power, after passing through an optional input line reactor, is rectified by a rectifier and capacitor to form a two-level DC bus. Depending on the design approach, input harmonics on the DC bus may be further reduced by a DC reactor. The two-level DC bus voltage is applied across a six-switch inverter bridge which produces a two-level PWM voltage output. The six switches are divided into three branches with two switches each. A controller controls each switch via the control terminals of each switch. A three-phase motor has a phase connection derived from the middle point between the two switches of a branch, and the three branches produce three phases which collectively drive the motor. The two levels of the DC bus constitute a positive bus and a negative bus. The top switch of each branch is connected to the positive bus and the bottom switch of each branch is tied to the negative bus. The two switches in a branch are in series and therefore cannot be turned-on at the same time without causing a short-circuit. In order to prevent a short-circuit, switch delay times are taken into consideration by the controller. The top switch needs to turn-off before the bottom one turns-on, and vice-versa. Each of the switches has to be able to handle the full voltage between the positive and negative busses.
In comparison to the two-level drive, in a three-level AC drive, the DC bus has three voltage levels (relatively labeled positive, neutral and negative), and the inverter bridge has twelve switches. The switches are divided into three equal branches, each branch connecting to one phase of the three-phase motor. Thus, each branch has four switches in series, and each connection to the motor is derived from a middle point.
A drawback of a three-level inverter bridge is that while a two-level inverter bridge requires only six semiconductor power switches, a three-level inverter bridge requires twelve switches, thereby increasing costs. These costs continue to increase as additional levels are utilized. For example, a four-level inverter bridge requires eighteen switches and a five-level inverter bridge requires twenty-four switches.
Further increasing costs result since as the number of levels and switches in the inverter bridge is increased, the complexity of controlling the switches also increases. The signals that drive the switches need to be carefully timed; otherwise, the switches may be damaged or destroyed. This complexity increases the costs of controllers used with multiple-level inverter bridges.
Known multi-level voltage source inverters employ two cables for activation of each power semiconductor device (e.g., without limitation, an IGBT). For example, these cables can be copper, but are typically fiber optic cables for noise immunity and isolation. One cable is employed for a signal to activate/fire the semiconductor device, and the other cable is employed to receive an activation response signal.
As the number of levels and devices increase, so does the number of wires or fibers. This problem worsens when multiple inverter bridges are employed for paralleling inverters, for redundant inverters, and in other situations where multiple inverters are employed. In a typical three-level drive, twenty-four fiber optic cables are employed for controlling and monitoring each inverter bridge. When two inverter bridges are employed, the number of fiber optic cables doubles. Furthermore, these fiber optic cables cannot transmit any data other than the firing pulses and the activation response signals for the inverter bridge.
FIG. 1 shows an inverter control and monitoring system 2 including a controller 4 having a controller module 5 and an inverter 6, such as a three-level inverter bridge. The inverter 6 is powered by a rectifier/capacitor DC bus 8 and powers a three-phase motor 10. The inverter 6 includes an IGBT assembly 12, which in this example includes twelve power semiconductor devices, such as IGBTs 14. Three example temperature sensors, such as the example RTDs 16, monitor temperatures of the IGBTs 14 for the three output phases. Although RTDs are disclosed, these may be another suitable temperature sensor, such as a thermocouple, or another suitable sensing device, not necessarily temperature related. Two isolated voltage sensors 18 monitor the voltage of the DC bus 8. Three current sensors, such as Hall current sensors 20, monitor the three-phase currents flowing between the inverter 6 and the motor 10. The sensed temperatures from the three RTDs 16 are communicated to the controller 4 by three or more cable connections 22. The sensed voltages from the two voltage sensors 18 are communicated to the controller 4 by two cable connections 24, and the sensed currents from the three Hall current sensors 20 are communicated to the controller 4 by three cable connections 26. Twelve control signals from the controller 4 to the IGBT assembly 12 are communicated using twelve fiber optic cables 28, and twelve monitoring signals to the controller 4 from the IGBT assembly 12 are communicated using twelve fiber optic cables 30. As such, there are 32 different connections between the controller 4 and the inverter 6.
Although a three-level inverter is discussed, the semiconductor devices 14 can be configured as, for example and without limitation, a two-level inverter or an inverter having four or more levels. The power semiconductor devices 14 can be, for example and without limitation, a plurality of IGBTs, a plurality of transistors, or other suitable power semiconductor electronic components.
There is room for improvement in systems controlling and monitoring power semiconductor devices.